The present invention relates to a novel and improved elastomer composition having excellent mold releasability, resistance to heat, solvents and oils, and mechanical, electric and other properties or, in particular, to an elastomer composition formulated on the base of an ethylene-propylene copolymeric elastomer having similar, excellent physical properties.
Ethylene-propylene copolymeric elastomers, such as the so-called EPR, i.e. ethylene-propylene rubbers, EPT, i.e. ethylene-propylene terpolymers, and EPDM, i.e. ethylene-propylene-diene terpolymers, have a considerably good weathering resistance and heat resistance resulting from the very low content of, or even absence of, unsaturated linkages in their molecular structure as well as excellent electric properties as a synthetic rubber. Demands and uses of these elastomers have rapidly grown in recent years owing to the advantages of the above-mentioned properties coupled with economical advantages in costs, compared to other synthetic rubbers.
However, the ethylene-propylene copolymeric rubbers are no better than nitrile rubbers and styrene-butadiene copolymeric rubbers in regard to resistance to oils and solvents, and definitely inferior when compared with silicone rubbers and fluorocarbon rubbers not only in oil and solvent resistance but also in mold releasability and heat resistance.
In order to compensate the above described defective properties of ethylene-propylene copolymeric elastomers, a method of polymer blend has been proposed in which those elastomers are blended with a silicone rubber having excellent resistance to oils, solvents, and heat as well as excellent mold releasability, despite some problems as described in the following.
Commercially available silicone rubbers are classified, in general, into three classes according to the mechanisms of crosslink formation. They are (1) those crosslinked with an organic peroxide, (2) those crosslinked by addition reaction between silicon-bonded vinyl groups and silicon-bonded hydrogen atoms with the catalytic action of a platinum catalyst, and (3) those crosslinked by the condensation reaction in the presence of a condensation catalyst such as metal salts of organic acids.
It is evident that the third type of silicone rubbers cannot be co-vulcanized with ethylene-propylene copolymeric elastomers in a polymer blend due to remoteness in the mechanisms of their crosslink forming reactions, since the latter elastomers are usually vulcanized with sulfur, a sulfur-containing vulcanizing agent or an organic peroxide.
The first and the second types of silicone rubbers are, on the other side, co-vulcanizable with ethylene-propylene copolymeric elastomers, if the matter is solely on the possibility of crosslink formation, since at least a small amount of double bonds are almost always introduced into the molecular structure of ethylene-propylene copolymeric elastomers by the copolymerization with a dienic comonomer, such as ethylidenenorbornene, dicyclopentadiene and 1,4-hexadiene, with the purpose of increasing the crosslinkability of the resultant elastomers.
Unfortunately, a polymer blend of a silicone rubber of the second type and an ethylene-propylene copolymeric elastomer has a very short pot life due to the addition reaction taking place even at room temperature in the presence of a platinum catalyst. In addition, the platinum catalyst is very sensitive to deactivation by poisoning with trace amounts of sulfur compounds, amine compounds, phosphorus compounds and certain compounds containing metals, such as lead, tin, zinc, bismuth, cobalt and the like. Thus polymer blends of ethylene-propylene copolymeric elastomers with silicone rubbers of the second type are of less practical importance.
On the other hand, a polymer blend of an ethylene-propylene copolymeric elastomer and a silicone rubber of the first type is also defective due to unbalance between the competitive reactions of the rapid crosslink formation of the silicone rubber with an organic peroxide and the relatively slow crosslink formation of the ethylene-propylene copolymeric elastomer with the same organic peroxide.
Thus the method of the polymer blend of ethylene-propylene copolymeric elastomers with silicone rubbers of any of the three types can not be free from serious disadvantages, not to mention the insufficient improvement of the desired properties.